The polyomaviruses comprise a class of small DNA tumor viruses within the papovavirus group of DNA viruses. Members of the polyomavirus class include polyoma virus (Py) of mice, simian virus 40 (SV40) of monkeys, and JC and BK viruses of humans. Of these viruses, Py has been most thoroughly characterized with respect to the genetic elements and proteins involved in oncogenic transformation of mammalian cells. Genetic and biochemical analysis has revealed that the region of the Py genome required for both Py-mediated tumorigenesis in vivo and oncogenic transformation in vitro encodes three nonstructural proteins: the large, middle, and small tumor antigens (T Ags). These proteins have been designated as T Ags because they are recognized by antibodies in sera from animals bearing Py-induced tumors (T sera). Of these three proteins, a central role for the Py-encoded middle T Ag (MTAg) in Py-mediated oncogenesis has been established principally through genetic analysis. The MTAg is associated with a protein kinase activity which can be detected by the in vitro phosphorylation of tyrosine on MTAg when T sera immunoprecipitates of Py-infected or Py-transformed cell extracts are incubated with gamma-32P ATP and Mg++. The MTAg apparently does not possess intrinsic protein kinase activity and is thought to associate with the cellular c-src gene product, the cellular homolog of the Rous sarcoma virus transforming gene. Since both the viral (pp60v-src) and cellular (pp60c-src) forms of the src gene protein possess intrinsic tyrosine protein kinase activity, it has been proposed that the MTAg associated protein kinase represents a property of pp60c-src. The potential importance of this protein kinase activity in Py-mediated oncogenesis is suggested by the finding that viral mutants which are deficient in transforming potential generally lack this activity and that there are no known tranformation-competent Py strains which encode MTAg molecules which do not possess this associated kinase activity.